Embodiments of the invention relate generally to electrical power generators, and more particularly to starting and running variable speed electrical power generators.
In this regard, many electrical generators are started by inducing current in a stator with a stator converter and inducing a current in a rotor with a rotor converter. The current from the stator converter interacts with flux in the machine to create torque in the generator. The torque turns the generator rotor and a prime mover of the generator. When the generator prime mover reaches a rated starting speed, fuel is ignited in the prime mover of the generator. Once the prime mover is operating, the generator may be aligned electrically for power generation.
During the start sequence of a generator, the stator converter sends current to the stator and a rotor converter sends current to the rotor. The relationship between the voltages and frequencies of the stator and rotor currents affect the flux in the generator and the torque induced by the stator to start the generator. In starting, the voltage in the stator is increased until the stator converter reaches a rated voltage and frequency limit. The voltage rating of the stator converter limits the flux induced in the generator, so that at some speed the flux must be reduced according to the voltage limit of the stator converter. This results in a limitation on the torque available for starting the generator.
Many electrical power generators use rotor converters that output DC current to the rotor. Since the DC current does not have a frequency, frequency of the rotor current cannot be adjusted. The limitation on adjusting the frequency of the rotor current limits the available starting torque that may be produced by the stator.
It is desirable to increase the available flux and torque in a generator stator during a start sequence.